The invention relates to a solenoid operation checking method and apparatus for checking whether solenoids such as various kinds of electromagnetic valves, electromagnetic plungers, or the like are functioning and, more particularly, to a solenoid operation checking method and apparatus in which a current is supplied to a solenoid coil and the solenoid coil is energized and, thereafter, the solenoid coil is temporarily deenergized and the operation is checked, the current supply to the solenoid coil is shut off, and after that, the coil is temporarily energized and the operation is checked.
For instance, as shown in FIG. 1, according to a conventional electromagnetic valve, a spool 14 is slidably provided in a slide hole 12 formed in a body 10, and the spool 14 is held at a closed valve position shown in the diagram by springs 22 and 24 provided in spring chambers 18 and 20 on both sides of the spool 14, that is, the spool 14 is held at a position where a portion between an inlet port 26 and an outlet port 28 is shut off by a land 16. When a current is supplied to a coil 32 of a solenoid 30 provided at one end of the body 10 and the coil 32 is energized, a movable iron core 34 is moved to the left by an attracting force between the movable iron core 34 and a fixed iron core (not shown) in accordance with a magnitude of the exciting current. The spool 14 is moved to the left by a push rod 36, thereby opening a portion between the inlet port 26 and the outlet port 28.
In FIG. 2, a reaction force of the spring 22 in FIG. 1 is shown by a solid line 38, a force which is obtained by adding a hydraulic power to the spring reaction force 38 is shown by an alternate long and short dashed line 40, and an output of the solenoid 30 is shown by a solid line 42. Reference numeral 43 denotes a position where the valve starts to open.
However, if the spool 14 causes a phenomenon such that it becomes fixed due to a choke of an alien matter or the like or if the movable iron core 34 of the solenoid 30 is closely adhered in a slide sleeve, even if the exciting current sufficient to obtain the solenoid output shown by the solid line 42 is supplied to the coil 32 at the fully closed position, the movable iron core 34 cannot be moved to the left. On the other hand, even if the exciting current is reduced so as to decrease the solenoid output by only an amount .DELTA.F (=spring force+ hydraulic power) at the fully opened position, it is possible that the movable iron core 34 will not move away from the adsorbing position.
Hitherto, as a method of checking the operating state of such a solenoid, for instance, there has been known a method whereby the operation of the solenoid at a time point when the solenoid has been turned off is checked on the basis of a difference of a current response waveform due to a moving situation of the movable iron core in the solenoid as disclosed in JP-A-1-265504.
The above method will be briefly explained. As shown in FIG. 3, when a current detecting resistor 44 is serially connected to the coil 32 of the solenoid and a current is supplied from a power source 46 to the series circuit by turning on a switch 48, a magnitude of an exciting current flowing through the coil 32 is converted into a voltage by the resistor and is detected. A current response waveform of a detection voltage e differs depending on whether the movable iron core is moved or not. Such a difference of the current response waveform is used.
That is, if the movable iron core had been moved and the solenoid has normally operated, as shown by a solid line 60 in FIG. 4B, the response waveform temporarily drops during the rising state. However, in an abnormal state in which the movable iron core doesn't move, the response waveform doesn't drop, as shown by an alternate long and short dashed line 62.
Therefore, the detection voltage e of the resistor 44 is amplified by an amplifier 50, a peak value is held by a peak holding circuit 52, and the peak value is subsequently compared with a new detection value by a comparator 54. Thus, an output of the comparator 54 is inverted to the "H" (high) level in the dropout waveform portion in the normal state.
The AND of an output of a comparator 56 which is set to the "H" level as shown in FIG. 4A by the turn-on of the switch 48 and an output of the comparator 54 is obtained by the AND circuit 58, so that a pulse output as shown in FIG. 4C is derived. The pulse output shown in FIG. 4C is not obtained in the normal state. Therefore, a discrimination regarding whether the solenoid has normally operated or not can be made by the presence or absence of the pulse output from the AND circuit 58.
However, in such a conventional method of checking the operation of the solenoid, the operation can be judged only at a moment when the solenoid has been turned on. For instance, in the case where an abnormality has erroneously been detected by the reception of noises at a moment when the solenoid had been turned on, the operator must operate the solenoid again in order to check the operation. However, it is impossible to execute the solenoid again because the reoperation of the solenoid causes a spool and an actuator to be moved. There is a problem in that the operation cannot be checked after the solenoid was operated.
The reliability for the result of the operation check is also low. Particularly, in the case of a shockless electromagnetic valve in which movement has been made difficult by filling an oil into the enclosing portion of a movable iron core, since the motion of the movable iron core is slow, a large difference of a response waveform doesn't appear and there is a problem on reliability of the judgment.
A number of other apparatuses of the type in which a discrimination regarding whether the movable iron core of the solenoid is moved or not function on the basis of a difference of the response waveform of a current or a voltage.
As another method of checking the solenoid operation, there have been proposed many methods of checking the operation of the solenoid by a detection signal by providing a mechanical limit switch, a contactless switch, potentiometer, or the like into the solenoid.
However, the mechanical structure is generally complicated if the above methods are used, and an additional space is needed in order to enclose such a switch or the like. In the case of the mechanical type switch, there is a problem of durability.
On the other hand, when the solenoid has been turned off, the operation is checked by an apparatus shown in FIG. 5.
As shown in FIG. 5, the current detecting resistor 44 is serially connected to the coil 32 of the solenoid. When the switch 48 is turned off from the on state in a state in which a current has been supplied from the power source 46 to the coil 32 by turning on the switch 48, a spike voltage of a polarity opposite to that of a power source voltage is generated by an energy accumulated in the coil 32 as shown in FIG. 6A. A voltage which is equal to or lower than a predetermined voltage is eliminated from the spike voltage by a limiter 150 and, thereafter, the spike voltage is amplified by the amplifier 50, so that a negative constant voltage is generated until time t.sub.2 as shown in FIG. 6B.
When the movable iron core starts to return at time t.sub.2, an impedance of the coil 32 changes and a convergence of the spike voltage is delayed. After the return of the movable iron core is completed at time t.sub.3, the impedance doesn't change, so that the spike voltage is rapidly converged toward zero.
A differentiation waveform output by a differentiating circuit 152 in FIG. 5, therefore, becomes a positive pulse waveform as shown in FIG. 6C. The pulse waveform is compared with a reference voltage V.sub.ref by a comparator 154. If a pulse signal shown in FIG. 6D is derived, this means that the solenoid has normally been turned off.
The AND of the above pulse signal and an output shown in FIG. 6E from an inverter 156 is obtained by an AND circuit 158 and is generated as a solenoid off detection signal.
However, even in such a conventional method of checking the operation at the time of turn-off of the solenoid, a check is made to see if the movable iron core has been returned to the initial position or not on the basis of a difference between a current discharge waveform when the movable iron core is moving upon turn-off of the solenoid and a current discharge waveform when the movable iron core is restricted and doesn't move. Consequently, the operation of the solenoid can be judged only at a moment when the solenoid has been turned off. There is also a problem regarding the reliability of the check similar to the case in which the solenoid is turned on.